The proposed study is directed toward evaluation of hematopoietic cellular proliferation in clinical disorders of blood associated with neoplasms, infection, polycythemia vera and other myeloproliferative disorders. The studies include 1) assay and characterization of regulatory factors (humoral or cell-mediated) present in these patients by the study of their in vivo effects in animals (mice and fetal sheep) as well as their influence on the proliferation and differentiation of animal and human bone marrow cells in vitro; 2) the evaluation of the proliferative characteristics of hematopoietic precursors by their response to physiologic regulators (erythropoietin etc.) and selected steroid metabolites in vitro; 3) the physical separation and characterization of erythroid precursors from bone marrow of patients with myeloproliferative disorders (and selected patients with anemias of infection and neoplasms) with capabilities of forming endogenous colonies in vitro and/or exerting inhibitory influences on erythroid colony formation by normal human marrow in vitro. Comparative studies will also be performed with fetal sheep hematopoietic cells; 4) investigation of mechanisms underlying the progressive change in fetal sheep erythroid precursors from relative independence from erythropoietin to total dependence upon exogenous erythropoietin; and 5) development of a more meaningful assay for the myelostimulatory factor present in sera of some patients with myeloproliferative syndrome.